Human and animal breath contains hundreds of different trace volatile organic compounds (VOCs), in addition to the usual large amounts of H2O and CO2. The metabolic pathways leading to the generation of these VOCs are mostly little understood. However, much effort has been recently expended correlating the presence of particular VOCs with particular diseases, and breath analysis may yet prove to be a useful and routine procedure for assisting clinicians.
A clinical procedure which currently makes use of breath analysis is the δ13C urea breath test. This test can be a helpful tool for clinicians seeking to diagnose the presence of a Helicobacter pylori infection in the human gut, which is commonly associated with gastric ulcers and carcinoma. A patient receives an oral dose of urea having a known enhanced level of the 13C isotope. Colonies of Helicobacter pylori, which secrete a urease enzyme, hydrolyse the [13C] urea to 13CO2 and ammonia. The 13CO2 enters the bloodstream and is subsequently exhaled. “δ13C” is a parts per thousand expression of the enhancement in the relative proportions of 13C and 12C in a sample over a standard or background level.
A common technique employed in the measurement of δ13C in exhaled breath is isotope ratio mass spectrometry (IRMS). This technique distinguishes between isotopomers of a molecular species, for example δ13CO2, by the mass/charge ratio of ions of the species. The technique is limited by the existence of two CO2 isotopomers with an atomic mass of 45, namely 13C16O2 and 12C16O17O, as well as by sample contamination with 12C16O2H. Furthermore, the technique generally requires high vacuums, low impurity levels, and expensive and bulky equipment.
A number of spectroscopic methods have been proposed as alternative techniques for determining δ13CO2. These methods exploit the differences in the distributions of rotational and vibrational energy states between 12CO2 molecules. A number of such techniques, including nondispersive and fourier transform infrared techniques are mentioned in “Precision Trace Gas Analysis by FT-IR Spectroscopy. 2. The 13C/12C Isotope Ratio of CO2”, M. B. Esler et al., Analytical Chemistry 72, No. 1, 2000.
Methods currently used for detecting volatile organic compounds in breath analysis were reviewed by W-H Cheng and W-J Lee in “Technology development in breath microanalysis for clinical diagnosis”, JL'ab Clin Med 133, No. 3, 1999. The techniques mentioned include gas chromatography, mass spectrometry, fourier transform and nondispersive infrared spectroscopy, the selected ion flow tube and surface acoustic wave techniques, chemiluminescence and colorimetry.
The techniques mentioned above have various disadvantages, particularly when an inexpensive, compact and robust apparatus for clinical use is sought. Mass spectrometry requires bulky and expensive equipment operating with high vacuums and voltages. Gas chromatography relies on the use of specially prepared separation capillaries, may be slow, and is insensitive to isotopic differences. The various infrared spectroscopic techniques are limited by very low IR absorption rates resulting from low concentrations of the target molecule in small experimental volumes, thereby requiring long experiment duration and expensive detectors and post processing circuitry to yield satisfactory results.